There are two version of the fuel gauge sender unit. The original one has no pickup, it's just a sender. With that one the banjo on top of the tank is connected to a steel tube that goes to within an 1/8" or so of the bottom of the tank in an indented well.
That is the tube that rusts out and is not replaceable.
There is a second type, which is currently serviced that has the pickup as part of the fuel gauge sender/float assembly. It's unlikely that a recent one will rust out so fast.
I had a dead sender assembly so I replaced the float assembly with one currently serviced by Wilkinson. That tube is steel and as I remember was just yellow cad plated.
That float also had to be "adjusted" quite a bit to have the gauge read anywhere near accurately, but that's a different story.
IF you have gone to the pickup integral to the float/sender unit, scientifically you need to eliminate a tube failure as a possibility, pull it out, and vacuum/pressure test it on the bench.
In the attached illustration, the tank on the left has the original design showing #1 as the fuel banjo. Below that, not shown in the diagram is the steel pickup tube.
The second fuel pickup illustration in the center of the diagram is the replacement sender/pickup with the integral pick up.
